When CERN’s gargantuan accelerator, the Large Hadron Collider (LHC), went off ten years ago, hopes abounded that new particles would soon be discovered that could help us unravel the deepest mysteries of the Earth. physical. Dark matter, microscopic black holes and hidden dimensions were just a few of the possibilities. But other than the spectacular discovery of the Higgs boson, the project gave no clue as to what might lie beyond the Standard Model of particle physics, our current best theory of the micro-cosmos.
So our new document from LHCb, one of the four giant experiments at the LHC, is likely to make physicists’ hearts beat a little faster. After analyzing the billions of collisions produced over the past decade, we can see evidence of something entirely new – potentially the bearer of a whole new force of nature.
But the excitement is tempered by extreme caution. The Standard Model has withstood all the experimental tests launched since it was assembled in the 1970s, so claiming we’re finally seeing something it can’t explain requires extraordinary evidence.
The Standard Model describes nature on the smallest scale, comprising fundamental particles called leptons (such as electrons) and quarks (which can come together to form heavier particles such as protons and neutrons) and the forces with which they interact with.
There are many types of quarks, some of which are unstable and can decay into other particles. The new result is linked to an experimental anomaly that was first mentioned in 2014, when physicists at the LHCb spotted “beauty” quarks disintegrating unexpectedly.
Specifically, beauty quarks seemed to decay into leptons called “muons” less often than they decay into electrons. This is strange because the muon is essentially a carbon copy of the electron, identical in every way except that it is about 200 times heavier.
One would expect beauty quarks to decay to muons as often as they do to electrons. The only way these decays could occur at different rates is if particles never seen before were involved in the decay and tip the balance against muons.
While the 2014 result was intriguing, it wasn’t specific enough to draw a firm conclusion. Since then, a number of other anomalies have appeared in related processes. They were all individually too subtle for researchers to be convinced these were true signs of a new physics, but enticingly, they all seemed to point in a similar direction.
The big question was whether these anomalies would get stronger as more data was analyzed or melted into nothing. In 2019, LHCb again performed the same beauty quark decay measurement, but with additional data taken in 2015 and 2016. But things weren’t much clearer than they had been five years ago. earlier.
Today’s result doubles the existing dataset by adding the sample recorded in 2017 and 2018. To avoid accidentally introducing bias, the data was analyzed “blind” – scientists could not see the result until all the procedures used in the measurement have been tested and revised.
Mitesh Patel, a particle physicist at Imperial College London and one of the leaders of the experiment, described the excitement he felt when the time came to look at the result. “I was actually shaking,” he said, “I realized this was probably the most exciting thing I’ve done in 20 years in particle physics.”
When the result appeared on screen, the anomaly was still there – about 85 muon decays per 100 electron decays, but with lower uncertainty than before.
What will fascinate many physicists is that the uncertainty of the result is now greater than “three sigma” – the way scientists say that there is only one chance in a thousand that the result is a fluke. random data. By convention, particle physicists call anything above three sigma “proof.” However, we are still a long way from a confirmed “discovery” or “sighting” – which would require five sigma.
Theorists have shown that it is possible to explain this anomaly (and others) by acknowledging the existence of completely new particles that influence the way quarks decay. One possibility is a fundamental particle called “Z prime” – essentially the bearer of a whole new force of nature. This force would be extremely weak, which is why we have not seen any signs of it so far, and would interact differently with electrons and muons.
Another option is the hypothetical ‘leptoquark’ – a particle that has the unique ability to decay into quarks and leptons simultaneously and that could be part of a larger puzzle that explains why we see the particles we make in nature. .
Interpret the results
So have we finally seen evidence of a new physics? Well, maybe, maybe not. We do a lot of measurements at the LHC, so you can expect at least some of them to fall as far from the standard model. And we can never totally rule out the possibility that there is a bias in our experience that we have not properly accounted for, even though this result has been verified extremely thoroughly. Ultimately, the picture will only become clearer with more data. LHCb is currently undergoing a major upgrade to dramatically increase the collision registration rate.
Even if the anomaly persists, it will likely not be fully accepted until an independent experiment confirms the results. An interesting possibility is that we might be able to detect the new particles responsible for the effect created directly in collisions at the LHC. Meanwhile, the Belle II experiment in Japan should allow similar measurements to be made.
So what could this mean for the future of fundamental physics? If what we are seeing is truly the harbinger of some new fundamental particles, it will ultimately be the breakthrough physicists have been striving for for decades.
We’ll finally have seen part of the big picture that lies beyond the standard model, which may ultimately allow us to unravel a number of established mysteries. These include the nature of the invisible dark matter that fills the universe or the nature of the Higgs boson. It might even help theorists unify particles and fundamental forces. Or, maybe best of all, it could indicate something we never even considered.
So, should we be excited? Yes, results like this don’t happen very often, the hunt is definitely on. But we must be careful and humble too; extraordinary claims require extraordinary evidence. Only time and hard work will tell if we have finally seen the first glimmer of what lies beyond our current understanding of particle physics.
This article by Harry Cliff, particle physicist, University of Cambridge; Konstantinos Alexandros Petridis, Lecturer in Particle Physics, University of Bristol, and Paula Alvarez Cartelle, Lecturer in Particle Physics, University of Cambridge, are republished from The Conversation under a Creative Commons license. Read the original article.